I installed silverlight on my mac and experienced continuous browser crashes both FF and Safari. I removed silverlight and still have the same problem. I have not had a crash in the three years previous to the installation.
A bit late in the game, but Silverlight plugin defenitly causes Safari & Firefox to crash. I had this problem again even with the new release version. You can easily duplicate the problem: Use your email on Windows Live (you may need to register if you don't have one), go to send or forward a message, as soon as you type the first letter of an name (in your address book) the browser (Safari or Firefox) will crash. It does not happen however when using Opera! To rectify the problem, remove Sulverlight plugin.